1. Field of the Invention
This invention relates to a Pr-doped inorganic compound, such as a Pr-doped garnet type compound. This invention also relates to a luminescent composition and a luminescent body, each of which contains the Pr-doped inorganic compound. This invention further relates to a light emitting device, a solid laser device, and an ionizing radiation detecting device, each of which utilizes the luminescent body.
2. Description of the Related Art
As inorganic compounds, which are capable of being exited by irradiation of exciting light and are thereby capable of producing luminescence, there have heretofore been known the inorganic compounds containing rare earth element ions as luminescence center ions. As for Pr, which is one of the rare earth elements, it has been known that Pr exhibits a plurality of luminescence (fluorescence) peaks in a wide wavelength range of an ultraviolet region to an infrared region, and it has therefore been thought that Pr is useful as a luminescent material.
Examples of matrix compounds to be doped with Pr include halides, such as fluorides, oxyhalides, chalcogenides, and oxychalcogenides. The compounds enumerated above are chemically unstable and have the problems with regard to the production cost in that, for example, large-scale production equipment is required. Therefore, the compounds enumerated above are not appropriate for the matrix compounds to be doped with Pr.
As the matrix compounds to be doped with Pr, oxides, such as garnet type compounds, which are chemically stable and are capable of being produced at a low cost, are preferable. In, for example, “Visible Laser Emission of Pr3+ in Various Hosts”, M. Malinowski et al., Journal de Physique IV, pp. C4-541-C4-544, 1994, an inorganic compound (Pr:YAG), in which Y3Al5O12 (YAG) that is one of the garnet type compound is utilized as the matrix compound, and in which Pr has been doped into the matrix compound YAG, is described. In the aforesaid literature, it is reported that Pr:YAG produces a blue laser beam (487.9 nm, at most 32K) and an orange laser beam (616 nm, at most 140K) when being excited (in this case, pumped) by a dye laser beam having a wavelength of 480 nm at low temperatures.
However, it has heretofore been stated that it is not always possible to form a solid solution of Pr in YAG. Specifically, in cases where Pr is to be doped in YAG, a part of y3+ ions at an A site are substituted by Pr3+ ions through the formation of the solid solution. However, an ionic radius (=0.1126 nm) of the Pr3+ ions (at the A site) is larger than the ionic radius (=0.1019 nm) of the Y3+ ions (at the A site). Therefore, a coefficient of segregation at the time of the doping of Pr in YAG is approximately equal to zero (as described in, for example, “Development and Prospect of Ceramic Laser Elements”, A. Ikesue et al., Laser Review, Vol. 27, No. 9, pp. 593-598, 1999.) The foregoing indicates that it is not always possible to form the solid solution of Pr in YAG. FIG. 20 is a graph showing relationships between ionic radiuses of rare earth element ions, which are to be doped in YAG, and segregation coefficients of the rare earth element ions.
In, for example, “Synthesis of Pr Heavily-Doped, Transparent YAG Ceramics”, A. Ikesue and Y. Sato, Journal of the Ceramic Society of Japan, Vol. 109, No. 7, pp. 640-642, 2001, there is described that, in the cases of a single crystal, it is not always possible to produce Pr:YAG, in which a Pr doping concentration is higher than 1 mol %.
As for a poly crystal sintered body, in, for example, the aforesaid “Synthesis of Pr Heavily-Doped, Transparent YAG Ceramics”, A. Ikesue and Y. Sato, Journal of Ceramic Society of Japan, Vol. 109, No. 7, pp. 640-642, 2001, a report is made on a compound (4.3% Pr-YAG), in which Pr is doped at a concentration of 4.3 mol % in YAG. However, as described above, the difference in ionic radius between the Y3+ ions, which act as the substitutable ions, and the Pr3+ ions, which act as the substituent ions, is large. Therefore, there is the possibility that a lattice strain will occur at positions in the vicinity of the Pr3+ ions, and that oxygen defects will occur. Since a lattice strain and lattice defects will cause deactivation of excited photons to occur, there is the risk that the lattice strain and the lattice defects will adversely affect the luminescence characteristics, such as a fluorescence intensity.
As a Pr-doped inorganic compound, in which an oxide other than the garnet type compounds is utilized as the matrix compound, a compound, in which YAlO3 is utilized as the matrix compound, and in which Pr is doped in YAlO3, is described in, for example, “CW-Lasing of Pr: YAIO3 at Room Temperature”, A. Bleckmann et al., OSA Proceedings on Advanced Solid-State Lasers, Vol. 15, pp. 199-201, 1993. In the literature described above, it is reported that the compound, in which Pr is doped in YAlO3, produces laser beams having wavelengths ranging from a red region to a near infrared region when being excited by an Ar laser beam having a wavelength of 476.5 nm. However, the compound described in the aforesaid literature is of the system of substitutable ions Y3+/substituent ions Pr3+ and is in the same circumstances as those described above with respect to Pr:YAG.
As a Pr-doped inorganic compound, in which the substitutable ions are other than the y3+ ions, a compound (Pr:Lu3Al5O12), in which Pr is doped in Lu3Al5O12, is reported as a scintillator material in, for example, “Photo- and Radioluminescence of Pr-Doped Lu3Al5O12 Single Crystal”, M. Nikl et al., Phys. Stat. Sol. (a), Vol. 202, No. 1, pp. R4-R6, 2005. The ionic radius of the Lu3+ ions (at the A site) is equal to 0.977 nm. Since the ionic radius of the Lu3+ ions is smaller than the ionic radius of the Y3+ ions, the difference in ionic radius between the substitutable ions and the substituent ions is larger than the difference in ionic radius in the cases of Pr:YAG, and therefore it is thought that the Pr doping in Lu3Al5O12 will be more difficult than in the cases of Pr:YAG.